A belt drive generally comprises an endless belt and at least two pulleys, of which one can operate as a drive and one can operate as an output of the belt drive. Such belt drives are especially used on combustion engines of a motor vehicle for driving accessories, wherein a first pulley is connected to the crankshaft of the combustion engine and drives the belt. Further pulleys are assigned to the accessories, like for example water pump, alternator or air conditioning compressor, and are rotatingly driven by the belt drive. In common belt drives, the accessories are configured as consumers, i.e. they are driven by the pulley of the crankshaft via the belt. In this case, an undriven portion (slack side) of the belt is formed between the crankshaft and the accessory adjoining the crankshaft in circumferential direction of the belt, said adjoining accessory typically being a generator. To ensure a sufficient wrap of the belt around the pulley, the belt is pretensioned by a tensioning roller of the belt tensioning device.
From DE 10 2011 053 869 A1 a belt tensioning device with exactly one tensioning arm is known, which is supported via a coil spring relative to the base member in a circumferential direction.
Also belt drives are known, in which a starter generator is integrated in the belt drive as further accessory, i.e., an electric motor, which depending on the operational condition can be operated as a starter or as an electric generator. In such belt drives with starter generator as an accessory between the engine operation on the one hand and the starter operation on the other hand a change between the driving portion and the driven portion takes place to both sides of the pulley of the starter generator. Thus it is necessary to provide spring loaded tensioning rollers for both of the named portions, of which respectively one is acting on the driven portion of the belt by means of the spring force, while the other is pushed away from the tensioned portion of the belt.
From DE 2013 102 562 A1, such a belt tensioning device with two tensioning arms is known, which are supported on each other by a spring in circumferential direction. The spring has a number of at least 1.25 and at maximum 2.5 windings.
From EP 2 128 489 A2 a belt tensioning device for a belt drive comprising a starter-generator is known. The belt tensioning device has a housing, in which two tensioning arms are pivotably supported around a common pivot axis. The tensioning arms are supported on each other by spring means. The housing is mountable in a drive pulley mounted on the starter-generator such that the housing is free of contact relative to the starter-generator in an annular portion enclosing the drive shaft of the starter-generator.
From US 2008/0070730 A1 a tensioner for tensioning a flexible drive member is known. The tensioner comprises a curve-shaped spring spine and wheels to engage the belt.
The space availability around the generator axis as well as in front of and behind the belt plane is especially limited for two tensioning devices or double arm tensioners. Furthermore, mounting of the belt is difficult not at least because of the space conditions. Depending of the design of the belt tensioner, high resulting torques may act on the belt tensioner, which may lead to an increased wear. Furthermore, the expenditure for manufacturing the spring for the belt tensioner is very cumbersome.